Figure 4-6 Low-Resistance Testing - Keithley 7015-S Instruction Manual

Low-level resistance measurements
Many times, it is necessary to make resistance measurements
with either lower voltage sensitivity or higher currents than
are available with ordinary DMMs. Examples of cases where
low-level resistance measurements may be necessary include
the testing of PC board traces, contacts, bus bars, and low-re-
sistance shunts.
Figure 4-6 shows a typical test configuration for a switching
system capable of testing a number of low resistance devices.
The Model 220 Current Source forces current through the
device under test, while the Model 182 Sensitive Digital
Voltmeter measures the resulting voltage across the device.
Since low voltage levels are being measured, thermal EMF
offsets generated by relay and connector contacts will have a
KEITHLEY
Figure 4-6
Low-resistance testing
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182 SENSITIVE DIGITAL VOLTMETER
TRG
SRQ
REM
TALK
LSTN
182 Input
Model 182
220 Output
220 Current Source
182
Nanovoltmeter
V
220 Currrent
Source
detrimental effect on measurement accuracy unless steps are
taken to avoid them. Thermal EMF effects can be virtually
eliminated by taking two voltage measurements, E
the first with the current, I, flowing in one direction, and the
second with a current, I, of the same magnitude flowing in
the opposite direction. The resistance can then be calculated
as follows:
R =
Note that simply reversing the current source polarity will re-
sult in a 2 × accuracy specification change. To avoid this
problem, matrix switching could be added to the test system
to reverse the current. See paragraph 4.4.4.
7015
HI
1
20
LO
HI
1
LO
20
Dual 1x20 MUX
R
7015
DUT
Operation
and E
1
E – E
2 1
--------------- -
2I
1
20
DUTs
(20)
4-9
,
2